A turboprop engine is a turbine engine that drives an aircraft propeller. Most of the engine's power is used to drive the propeller. The purpose of the propeller is to convert power, delivered by a rotating shaft, into thrust. It does this by accelerating a large mass of air to a higher velocity. The propeller itself is normally operated at a constant speed, with a variable pitch.
The efficiency of a propeller or a compressor is affected by several factors, one of them is the temperature. On cold days, the propeller or the compressor tend to rotate at a higher corrected speed while on hot days, the propeller or the compressor tend to rotate at a lower corrected speed. This causes changes in performance that would be desirable to recoup.